PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 515 



Mr. Johnson suggested, in removing rocks, to pump the sand 

 from helow them, and let them sink out of the way. There is a 

 tendency of sunken vessels to sink in the sand, if it is not very 

 dense, so that after a sufficient time they would of themselves go 

 down out of sight. 



Mr. Dibben was of opinion that this would not be the case. 

 There is the wreck of a vessel in our harbor now, that I have 

 seen there ten or fifteen years. Sometimes the sand will be 

 washed away, and sometimes will gather around it. 



Dr. Stevens. — I never had any fears that the Southern harbors 

 would be permanently obstructed by the sunken vessels. The 

 accumulation of water by the spring floods would be sufficient to 

 form new channels, so that we shall have to sink new vessels if 

 the war continues. It is for this reason that I am in favor of a 

 living blockade. 



JYew Subject. — The subject of " Coal Oils and Petroleum" was 

 selected for consideration at the next meeting. 



Adjourned. 



American Institute, Polytechnic Aissociation, 



February 20, 1862. 



Mr. S. D. Tillman was called to' the chair. 



The Sections not being fully prepared to report upon premiums, 

 were allowed another week. 



LARGE cannon. 



Dr. Rowell stated that it had been found, in casting large can- 

 non, that it is useless to exceed a certain thickness, on account 

 of the honeycomb structure which would result. To avoid this, 

 it had been proposed to cast the gun of a thickness which would 

 ensure tenacity, and to provide for the additional strength ; first, 

 by surrounding it with longitudinal stays of wrought iron, to 

 prevent bursting the end off, and next by shrinking on rings 

 of wrought iron. to resist the lateral pressure. The gun is cast 

 with projecting rings, between which sections of rings upon the 

 inside of the longitudinal stays would be placed, holding the 

 stays firmly in their position. He would propose, instead of cast- 

 ing the gun with the projecting rings, to cast it large enough to 

 form the rings by turning out grooves between them. 



Mr. C. W. Smith said that Captain Dahlgreen's experiments had 

 demonstrated that the angles upon the surface of a cast iron can- 



